Well I wouldn’t accept “I dunno” as an answer any more. “you are old enough now that it’s time for you to know because if you don’t people will take advantage of you”.
Every time they say ‘I dunno’ demand a single page report on the answer complete with at least 2 sources. No TV/Phone/Whatever until it is done. They’ll stop uttering that real quick. The key is co-participation though. Telling them something is important is meaningless when compared to showing them it is important by sitting down and helping them figureidout.
Seek a second opinion. That doctor is not listening to you. Something other than aging is going on and it should be properly investigated.
edit: is severe arthritis in your family (or any other bone/joint disease)? Are you exposed to large or long term doses of chemical(s) or radiation in your life through work or living environment (industrial zone nearby/upwind, or very old home). These are things I’d be considering.